Electrochemical measuring cells are widely used in the analysis of substances, for which potentiometry, voltammetry/polarography, coulometry and conductometry can be mentioned as the most important principles of measurement. The use of electrochemical measuring cells for the analysis of gases has been known for a long time as well. However, there are efforts to develop new, more sensitive and more reliable sensors, especially when toxic gases in the parts per billion (ppb) range are to be detected and possibly even quantitatively determined.
For the purpose of highly sensitive analysis, such electrochemical gas detectors are required to meet the following requirements:                low residual current I0;        no or at most only very slight effect of variations in the humidity of the air and/or the air temperature on the residual current I0;        low cross sensitivity to interfering gases;        low double-layer capacity of the measuring electrode, especially in relation to dynamic measurement methods; and        high long-term stability.        
The properties of an electrochemical gas sensor are determined decisively by the material, the morphology and the layer thickness of the measuring electrode.
Platinum, gold or graphite are mentioned as the measuring electrode materials in, e.g., U.S. Pat. No. 3,795,589. Very many gases can be reacted directly, i.e., without a mediator, on the catalytically highly active precious metals platinum and gold. Therefore, it is often impossible to reach the desired selectivity. The long-term stability of the catalytically less active graphite electrodes is low and these electrodes show, depending on the electrode potential, a high cross sensitivity to NO and NO2.
DE 199 39 011 C1 discloses a sensor, whose measuring electrode consists of diamond-like carbon (DLC). DE 101 44 862 A1 pertains to a measuring electrode consisting of boron-doped diamond (BDD). These electrode materials usually require mediators, because most gases cannot be directly reacted on these electrodes.